The present invention relates to circuitry for simultaneously reading multiple tracks of an optical disk, and more specifically to digital read channel circuitry suitable for implementation as an integrated circuit.
Due to their high storage density, long data retention life, and relatively low cost, optical disks have become the predominant media format for distributing information. Large format disks, and more recently, DVD disks, have been developed for storing full length motion pictures. The compact disk (CD) format was developed and marketed for the distribution of musical recordings and has replaced vinyl records. High-capacity, read-only data storage media, such as CD-ROM and DVD-ROM, have become prevalent in the personal computer field, and the DVD format may soon replace videotape as the distribution medium of choice for video information.
Physically, the information bearing portion of an optical disk is a series of pits, or bumps, arranged to form a spiral track. Data is encoded in the length of individual pits and of the space between pits. A laser beam reflected off of the optical disk is modulated by the pits and spaces, and received by a detector which produces a similarly modulated electrical signal, or track data signal.
The track data signal is demodulated to recover digital information stored on the disk by observing the amplitude of the track data signal responsive to a data clock. The characteristics of the track data signals enable the data clock to be derived from the track data signal using phase locked loop (PLL) circuitry. Data is encoded such that if the amplitude of the track data signal is approximately the same from one sample to the next, the corresponding bit has a value of xe2x80x980xe2x80x99; and a value of xe2x80x981xe2x80x99, otherwise.
The linear density of the bits and the track pitch are fixed by the specification of the particular optical disk format. For example, CD disks employ a track pitch of 1.6 xcexcm having approximately 80 thousand channel bits per linear inch, while DVD employs a track pitch only about one-half as wide and having approximately 200 thousand channel bits per linear inch. Because in most previously known systems the data are read from the disk serially, i.e. one bit at a time, the maximum data transfer rate for an optical disk reader is determined by the rate at which the pits pass by the pickup assembly.
Most prior efforts at increasing data transfer rates focused on rotating the optical disk at higher speeds, thereby increasing the rate at which the pits pass by the pickup assembly. However, cost, heat, vibration, and other practical considerations limit maximum spindle speed to about 6000-7000 rpm.
A cost effective alternative to increasing the disk rotational speed to provide faster optical disk readers is to read multiple data tracks simultaneously, as described in commonly assigned U.S. Pat. No. 5,426,623 to Alon et al. In accordance with the methods and apparatus provided therein, a multi-beam optical disk reader is capable of achieving very high speeds when reading an optical disk. A seven beam reader, for example, which rotates the disk at 8xc3x97standard speed, would provide a data rate equivalent to a 56xc3x97drive. Thus, simultaneously reading multiple tracks of an optical disk provides significant increases in data reading rates at relatively low spindle speeds, as compared to optical systems that read a single track.
It should be noted that as used herein, a data track is a portion of the spiral data track of a typical optical disk which follows the spiral for one rotation of the disk. Thus, a drive capable of reading multiple data tracks simultaneously reads multiple such portions of the spiral track at once. For optical disks having concentric circular tracks, a data track would refer to one such circular track. For disks having multiple concentric spiral tracks, a data track would refer to one revolution of one of the concentric spiral tracks.
One method of reading multiple data tracks is to replicate circuitry for reading single tracks. That is the circuitry represented by FIG. 1. would be repeated for each track. So an optical disk reader capable of simultaneously reading ten tracks would include ten sensors, equalizers, slicers, low pass filters (LPF), PLL""s, samplers, and demodulators. This results in circuitry of increased size, complexity, and expense.
The presence of multiple PLLs may cause difficulties for the circuit designer. Combining such circuitry into a single integrated circuit (IC), may cause interference between the analog circuits, such as the voltage controlled oscillator (VCO) portion of the PLLs, which may adversely affect circuit operation. To minimize such interference, the VCOs need to be physically spaced apart from one another on the IC. This effectively limits the number of PLLs that may be located on a single IC, which has the practical effect of limiting the number of channels that may be processed by a single IC.
It would therefore be desirable to provide methods and apparatus for simultaneously reading multiple tracks of an optical disk.
It would also be desirable to provide methods and apparatus that minimize interference between circuits.
It also would be desirable to provide methods and apparatus for simultaneously reading multiple tracks of an optical disk that would be conducive to high levels of circuit integration without compromising circuit performance.
In view of the foregoing, it is an object of the present invention to provide methods and apparatus for simultaneously reading multiple tracks of an optical disk.
It is also an object of the invention to provide methods and apparatus for simultaneously reading multiple tracks of an optical disk that enable high levels of circuit integration, without compromising circuit performance.
These and other objects of the present invention are achieved by providing a digital read channel that eliminates the need for separate analog PLLs for each track. An optical disk reader constructed in accordance with the present invention includes analog to digital converters (ADCs) for sampling the analog track data signals and converting the samples to digital values. The digital values are then digitally processed to recover data that was stored on the optical disk.
A frequency detector analyzes the processed digital values to identify synchronization symbols and data frames therein. The length of the synchronization symbols and the interval between two successive synchronization symbols are used to establish a time base for processing the digital values. This enables the digital read channel circuitry to rapidly determine the frequency of a track data signal.
A digital PLL (DPLL) uses the frequency information to lock onto the frequency of the track data signal. The DPLL has wide band (WB) and narrow band (NB) modes. The WB mode is entered after frequency has been determined by the frequency detector. When the DPLL is locked onto the frequency of the track data signal it enters the NB mode. This mechanism enables the DPLL to lock onto a track data signal rapidly, yet retain a higher degree of noise immunity.
In an embodiment for reading multiple tracks simultaneously, the multiple tracks may be sampled at the same rate and preferably at the same time. Digital read channel circuitry may be provided for each track being read, or alternatively, the read channel circuitry may be time shared between two or more tracks.